1. Field of the Invention
This invention relates to a pattern-switching temperature control apparatus for a furnace having a plurality of zones therein for heat-treating works at different temperatures. More particularly, the invention relates to a temperature control apparatus for controlling the temperature distribution in the so-called tunnel furnace which maintains a specific pattern of temperature distribution in the longitudinal direction thereof so as to heat-treat works while moving them therethrough by a conveyor or the like, the temperature control apparatus being adapted to automatically switch the pattern of the temperature distribution in the furnace when the kind of works being heat-treated is changed.
2. Description of the Prior Art
FIG. 2 schematicaly shows a conventional tunnel furnace. The furnace 1 has a plurality of zones Z1, Z2, . . . , Zn, and each zone is heated by a heating element 2 such as an electric heater. The heating of the furnace is controlled zone by zone either continuously or intermittently by regulating means 3 such as thyristors or other suitable electromagnetic switches. Temperature detecting elements 4, such as thermocouples, are disposed in the individual zones so as to detect the actual zone temperatures and communicate the detected temperatures to controllers 28A, 28B, . . . , 28N which are associated with the furnace zones respectively. Each controller has an arithmetic-logic unit 27A which compares the thus communicated actual zone temperature against the set value on a temperature setting element 27 thereof, and the unit 27A generates a control signal 5 based on the result of the comparison.
Each control signal 5 is applied to the regulating means 3 of the associated furnace zone, so as to regulate the energy supply to the heating element 2 of the furnace zone from an energy source 2A until the control signal 5 is reduced to nil, i.e. until the actual zone temperature is brought to the set value of the zone temperature. In case that the regulating means 3 are thyristors or other electromagnetic switches and the power source 2A is an electric power source, the control signals 5 regulate the making and breaking of the electric circuits through the regulating means 3.
The above temperature control brings about an overall temperature distribution pattern in the furnace 1, such as that of the solid line curve Pa of FIG. 2. Works 6 are heat-treated as they are moved through the furnace 1 as shown by the arrow 30 while maintaining the desired temperature distribution pattern therein.
Application of the arrangement of FIG. 2 is not restricted to electric furnaces. For instance, when the furnace 1 is heated by gas, the arrangement can be similarly applied to it by using the heating element 2 made of a gas burner, the regulating means 3 made of a valve, and the energy source 2A formed of a gas supply source.
To heat-treat different kinds of works 6, the temperature distribution pattern in the furnace 1 must be adjustable depending on the shape, size, an other properties of the works 6. In the case of heat-treating a large variety of different works 6 in small quantity, the temperature distribution pattern in the furnace 1 must be adjusted or switched rather frequently. In conventional temperature controllers, the setting elements 27 of the controllers 28A through 28N are manually controlled to switch the temperature distribution patterns, for instance from that of the solid line curve Pa of FIG. 2 to that of the dotted line curve Pb thereof. However, such manual control of the prior art has shortcomings in that it is not only time-consuming but also inaccurate.
Further, in the conventional apparatus, the desired temperature distribution pattern in the furnace 1 cannot be obtained quickly after changing the set values of the setting elements 27 and a certain time lag is inevitable before the desired temperature distribution pattern is produced and stabilized in the furnace 1. Accordingly, a waiting period is required before forwarding works 6 into the furnace 1 each time the temperature distribution pattern is changed.